Catalyst and process for producing aromatic compounds from C2 -C6

ABSTRACT

A catalyst for converting C 2  to C 6  aliphatic hydrocarbons to aromatics is described. The catalyst contains a zeolite, an aluminum phosphate binder and a gallium component. Examples of zeolites which can be used are the ZSM family of zeolites, with ZSM-5 being a specific example. The catalyst is characterized in that it is tolerant to exposure to hydrogen at tempertures of about 500° to about 700° C. The catalyst&#39;s tolerance to hydrogen exposure is the result of treating the catalyst with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution at a temperature of about 50° to about 100° C. for a time of about 1 to about 48 hours, followed by calcination. A process for preparing the catalyst is also described.

FIELD OF THE INVENTION

The present invention relates to a catalyst and to a process using thecatalyst for the production of aromatic hydrocarbons via thedehydrocyclodimerization of C₂ -C₆ aliphatic hydrocarbons. The catalystis characterized in that it is tolerant to exposure to hydrogen at atemperature of about 500° to about 700° C.

BACKGROUND OF THE INVENTION

Dehydrocyclodimerization is a process in which aliphatic hydrocarbonscontaining from 2 to 6 carbon atoms per molecule are reacted over acatalyst to produce a high yield of aromatics and hydrogen, with a lightends byproduct, C₂ -C₄ recycle product and a trace C₄ ⁺ nonaromaticbyproduct. This process is well known and is described in detail in U.S.Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference.Typically, the dehydrocyclodimerization reaction is carried out attemperatures in excess of 500° C., using dual functional catalystscontaining acidic and dehydrogenation components. The acidic function isusually provided by a zeolite which promotes the oligomerization andaromatization reactions, while a non-noble metal component promotes thedehydrogenation function. One specific example of a catalyst disclosedin U.S. Pat. No. 4,746,763 consists of a ZSM-5 type zeolite, gallium anda phosphorus containing alumina as a binder.

The conditions used for the dehydrocyclodimerization reaction result inrapid catalyst deactivation which is believed to be caused by excessivecarbon formation (coking) on the catalyst surface. This coking tendencymakes it necessary to frequently perform catalyst regenerations. Inaddition, applicants have noted that the prior art catalyst can bedeactivated by exposure to hydrogen at temperatures greater than 500° C.Minimizing the deactivation caused by this hydrogen exposure is aparticular object of this invention.

Applicants' catalyst contains a zeolite, a gallium component and analuminum phosphate binder, but is characterized in that it is tolerantto hydrogen exposure at temperatures greater than 500° C. The ability ofthe catalyst of this invention to withstand extended exposure tohydrogen without significant loss of activity is achieved by treatingthe catalyst with an aqueous solution of a weakly acidic ammonium saltor a dilute acid solution. This treatment removes some aluminum andphosphorus (and small amounts of silicon) from the catalyst as evidencedby analysis of the wash water. It is believed that this treatmentremoves an aluminum/phosphorus species which has deleterious effects onthe catalyst when exposed to hydrogen at high temperatures. Since thecatalyst is exposed to such conditions during normal operation, theability to remove such a deleterious species results in the unexpectedresult of increased catalyst life.

SUMMARY OF THE INVENTION

As stated, the instant invention relates to a catalyst, a process forpreparing the catalyst and a process for using the catalyst. Thus, oneembodiment of the invention is a catalyst for converting C₂ to C₆aliphatic hydrocarbons to aromatics comprising a zeolite having a Si:Alratio greater than about 10 and a pore diameter of about 5-6 Å, agallium component and an aluminum phosphate binder, the catalystcharacterized in that it is tolerant to exposure to hydrogen at atemperature of about 500° to about 700° C.

Another embodiment of the invention is a process for preparing acatalyst for converting C₂ to C₆ aliphatic hydrocarbons to aromaticscomprising: a) forming particles from a mixture of a zeolite and analuminum phosphate binder; b) calcining said particles at a temperatureof about 500° to about 700° C. and for a time of about 1 to about 15hours; c) impregnating the calcined particles with a gallium salt; d)heating the gallium containing particles in hydrogen at a temperature ofabout 500° to about 700° C. for a time of about 1 to about 15 hours,followed by heating in an air/steam mixture at a temperature of about400° to about 700° C. for a time of about 1 to about 10 hours; e)treating the particles of step (d) with an aqueous solution of a weaklyacidic ammonium salt or a dilute acid solution at a temperature of about50° to about 100° C. for a time of about 1 to about 48 hours; and f)calcining the particles at a temperature of about 500° to about 700° C.for a time of about 1 to about 15 hours, thereby providing saidcatalyst.

Yet another embodiment of the invention is a process for converting C₂to C₆ aliphatic hydrocarbons to aromatic compounds comprising contactinga feed stream containing C₂ to C₆ aliphatic hydrocarbons with a catalystat dehydrocyclodimerization conditions to provide aromatic compounds,the catalyst comprising a zeolite having a Si:Al ratio greater thanabout 10 and a pore diameter of about 5-6 Å, a gallium component and analuminum phosphate binder, the catalyst characterized in that it istolerant to exposure to hydrogen at a temperature of about 500° to about700° C.

DETAILED DESCRIPTION OF THE INVENTION

As stated, this invention relates to a catalyst, a process for preparingthe catalyst and a process for using the catalyst. The catalyst of thepresent invention comprises a zeolite component, a binder component, anda gallium metal component. The zeolites which may be used are any ofthose which have a Si:Al ratio greater than about 10 and preferablygreater than 20 and a pore diameter of about 5 to 6 Angstroms. Specificexamples of zeolites which can be used are the ZSM family of zeolites.Included among this ZSM family are ZSM-5, ZSM-8, ZSM-11, ZSM-12 andZSM-35.

The preparation of these ZSM-type zeolites is well known in the art andgenerally are prepared by crystallizing a mixture containing an aluminasource, a silica source, an alkali metal source, water and a tetraalkylammonium compound or its precursor. The amount of zeolite present in thecatalyst can vary considerably but usually is present in an amount fromabout 30 to about 90 weight percent and preferably from about 50 toabout 70 weight percent of the catalyst.

A second component of the catalyst of this invention is a phosphoruscontaining alumina (hereinafter referred to as aluminum phosphate)component. The phosphorus may be incorporated with the alumina in anyacceptable manner known in the art. One preferred method of preparingthis aluminum phosphate is that described in U.S. Pat. No. 4,629,717which is incorporated by reference. The technique described in the '717patent involves the gellation of a hydrosol of alumina which contains aphosphorus compound using the well-known oil drop method. Generally thistechnique involves preparing a hydrosol by digesting aluminum in aqueoushydrochloric acid at reflux temperatures of about 80° to 105° C. Theratio of aluminum to chloride in the sol ranges from about 0.7:1 toabout 1.5:1 weight ratio. A phosphorus compound is now added to the sol.Preferred phosphorus compounds are phosphoric acid, phosphorous acid andammonium phosphate. The relative amount of phosphorus and aluminumexpressed in molar ratios ranges from about 1:1 to 1:100 on an elementalbasis.

The resulting aluminum phosphate hydrosol mixture is now gelled. Onemethod of gelling this mixture involves combining a gelling agent withthe mixture and then dispersing the resultant combined mixture into anoil bath or tower which has been heated to elevated temperatures suchthat gellation occurs with the formation of spheroidal particles. Thegelling agents which may be used in this process are hexamethylenetetraamine, urea or mixtures thereof. The gelling agents release ammoniaat the elevated temperatures which sets or converts the hydrosol spheresinto hydrogel spheres. The spheres are then continuously withdrawn fromthe oil bath and typically subjected to specific aging and dryingtreatments in oil and in ammoniacal solution to further improve theirphysical characteristics. The resulting aged and gelled particles arethen washed and dried at a relatively low temperature of about 93° C. toabout 149° C. (200°-300° F.) and subjected to a calcination procedure ata temperature of about 450° C. to about 703° C. (850°-1300° F.) for aperiod of about 1 to about 20 hours. The amount of phosphorus containingalumina component present (as the oxide) in the catalyst can range fromabout 10 to about 70 weight percent and preferably from about 30 toabout 50 weight percent.

The zeolite and aluminum phosphate binder are mixed and formed intoparticles by means well known in the art such as gellation, pilling,nodulizing, marumerizing, spray drying, extrusion or any combination ofthese techniques. A preferred method of preparing the zeolite/aluminumphosphate support involves adding the zeolite either to an alumina solor a phosphorus compound, forming a mixture of the aluminasol/zeolite/phosphorus compound which is now formed into particles byemploying the oil drop method described above. The particles arecalcined as described above to give a support.

Another necessary component of the instant catalyst is a galliumcomponent. The gallium component may be deposited onto the support inany suitable manner known to the art which results in a uniformdispersion of the gallium. Usually the gallium is deposited onto thesupport by impregnating the support with a salt of the gallium metal.The particles are impregnated with a gallium salt selected from thegroup consisting of gallium nitrate, gallium chloride, gallium bromide,gallium hydroxide, gallium acetate, etc. The amount of gallium which isdeposited onto the support varies from about 0.1 to about 5 weightpercent of the finished catalyst expressed as the metal.

The gallium compound may be impregnated onto the support particles byany technique well known in the art such as dipping the catalyst into asolution of the metal compound or spraying the solution onto thesupport. One preferred method of preparation involves the use of a steamjacketed rotary dryer. The support particles are immersed in theimpregnating solution contained in the dryer and the support particlesare tumbled therein by the rotating motion of the dryer. Evaporation ofthe solution in contact with the tumbling support is expedited byapplying steam to the dryer jacket. After the particles are completelydry, they are heated under a hydrogen atmosphere at a temperature ofabout 500° to about 700° C. for a time of about 1 to about 15 hours.Although a pure hydrogen atmosphere is preferred to reduce and dispersethe gallium, the hydrogen may be diluted with nitrogen. Alternatively,it is envisioned that the reduction and dispersion can be done in situin the actual reactor vessel used for dehydrocyclodimerization by usingeither pure hydrogen or a mixture of hydrogen and hydrocarbons. Next thehydrogen treated particles are heated in air and steam at a temperatureof about 400° to about 700° C. for a time of about 1 to about 10 hours.The amount of steam present in the air varies from about 1 to about 40percent.

These catalyst particles which now contain well dispersed galliumpresent as gallium oxide are treated with an aqueous solution of aweakly acidic ammonium salt or a dilute acid solution. The ammoniumsalts which can be used include ammonium chloride, ammonium acetate andammonium nitrate. The concentration of these salts can vary from about0.1 to about 5 molar. The acids which can be used include hydrochloric,acetic, nitric and sulfuric acid. Although concentrated acids could beused, they would degrade the zeolite and the integrity of the particlesas well as removing the undesirable aluminum phosphorus species. Thus,it is desirable to use dilute acids which have a molarity from about 0.1to about 5 moles/liter. Of these treatment solutions, it is preferred touse an ammonium nitrate solution.

The treating solution is contacted with the calcined catalyst particlesat a temperature of about 50° to about 100° C. for a time of about 1 toabout 48 hours. After this treatment, the particles are separated fromthe aqueous solution, dried and calcined at a temperature of about 500°to about 700° C. for a time of about 1 to about 15 hours, therebyproviding the catalyst of the instant invention.

The purpose of treating the support with one of the solutions describedabove is to remove materials which cause the catalyst to deactivate whenit is exposed to hydrogen (hydrogen is produced during thedehydrocyclodimerization process) at temperatures above 500° C. andspecifically temperatures between 500° and 700° C. The exact nature ofthe species which is removed by this treatment step is not known.Without wishing to be bound by a particular theory, it is postulatedthat the deleterious species which is removed is an aluminum/phosphorusspecies. This hypothesis is based on the analysis of the wash waterafter the catalyst has been treated. The wash water also contains smallamounts of silicon indicating that the deleterious species may alsocontain silicon.

Although the exact nature of the species which is removed is not known,it is observed that treating a catalyst which contains an aluminumphosphate binder with one of the solutions described above renders thecatalyst tolerant to hydrogen exposure at temperature above 500° C. Bytolerant is meant that the catalyst can operate for a much longer periodof time without significant loss in activity. In the instant case, thetreated catalyst has a lifetime which is at least 6 times longer than acatalyst which has not been treated according to this invention.

The dehydrocyclodimerization conditions which will be employed for usewith the catalyst of the present invention will, of course, varydepending on such factors as feedstock composition and desiredconversion. A desired range of conditions for thedehydrocyclodimerization of C₂ -C₆ aliphatic hydrocarbons to aromaticsinclude a temperature from about 350° C. to about 650° C., a pressurefrom about 1 to about 20 atmospheres, and a liquid hourly space velocityfrom about 0.2 to about 5 hr⁻¹. The preferred process conditions are atemperature in the range from about 400° to about 550° C., a pressure inor about the range from 2 to 10 atmospheres and a liquid hourly spacevelocity of between 0.5 to 2.0 hr⁻¹. It is understood that, as theaverage carbon number of the feed increases, a temperature in the lowerend of the temperature range is required for optimum performance andconversely, as the average carbon number of the feed decreases, thehigher the required temperature.

The feed stream to the dehydrocyclodimerization process is definedherein as all streams introduced into the dehydrocyclodimerizationreaction zone. Included in the feed stream is the C₂ -C₆ aliphatichydrocarbon. By C₂ -C₆ aliphatic hydrocarbons is meant one or more open,straight or branched chain isomers having from two to six carbon atomsper molecule. Furthermore, the hydrocarbons in the feedstock may besaturated or unsaturated. Preferably, the hydrocarbons C₃ and/or C₄ areselected from isobutane, normal butane, isobutene, normal butene,propane and propylene. Diluents may also be included in the feed stream.Examples of such diluents include hydrogen, nitrogen, helium, argon,neon.

According to the present invention, the feed stream is contacted withthe instant catalyst in a dehydrocyclodimerization reaction zonemaintained at dehydrocyclodimerization conditions. This contacting maybe accomplished by using the catalyst in a fixed bed system, a movingbed system, a fluidized bed system, or in a batch type operation;however, in view of the danger of attrition losses of the valuablecatalyst and of the well-known operational advantages, it is preferredto use either a fixed bed system or a dense-phase moving bed system suchas shown in U.S. Pat. No. 3,725,249.

In a fixed bed system or a dense-phase moving bed the feed stream ispreheated by any suitable heating means to the desired reactiontemperature and then passed into a dehydrocyclodimerization zonecontaining a bed of the instant catalyst. It is, of course, understoodthat the dehydrocyclodimerization zone may be one or more separatereactors with suitable means therebetween to assure that the desiredconversion temperature is maintained at the entrance to each reactor. Itis also important to note that the reactants may be contacted with thecatalyst bed in either upward, downward, or radial flow fashion with thelatter being preferred. In addition, the reactants are in the vaporphase when they contact the catalyst. The dehydrocyclodimerizationsystem then preferably comprises a dehydrocyclodimerization zonecontaining one or more fixed or dense-phase moving beds of the instantcatalyst. In a multiple bed system, it is, of course, within the scopeof the present invention to use the present catalyst in less than all ofthe beds with another dehydrocyclodimerization or similarly behavingcatalyst being used in the remainder of the beds. Thisdehydrocyclodimerization zone may be one or more separate reactors withsuitable heating means therebetween to compensate for any heat lossencountered in each catalyst bed. Specific to the dense-phase moving bedsystem, it is commmon practice to remove catalyst from the bottom of thereaction zone, regenerate it by conventional means known to the art, andthen return it to the top of the reaction zone.

The following examples are presented in illustration of this inventionand are not intended as undue limitations on the generally broad scopeof the invention as set out in the appended claims.

EXAMPLE 1

This example describes the preparation of a dehydrocyclodimerizationcatalyst according to the prior art. A first solution was prepared byadding phosphoric acid to an aqueous solution of hexamethylenetatraamine(HMT) in an amount to yield a phosphorus content of the finishedcatalyst equal to about 11 weight percent. A second solution wasprepared by adding a ZSM-5 type zeolite to enough alumina sol, preparedby digesting metallic aluminum in hydrochloric acid, to yield a zeolitecontent in the finished catalyst equal to about 67 weight percent. Thesetwo solutions were commingled to achieve a homogeneous admixture of HMT,phosphorus, alumina sol, and zeolite. This admixture was dispersed asdroplets into an oil bath maintained at about 93° C. The dropletsremained in the oil bath until they set and formed hydrogel spheres.These spheres were removed from the oil bath, water washed, air dried,and calcined at a temperature of about 482° C. A solution of galliumnitrate was utilized to impregnate the spheres to achieve a galliumcontent on the finished catalyst equal to about 1 weight percent. Afterimpregnation, the spheres were dried, then heated in pure hydrogen at580° C. for 6 hours. The spheres were finally calcined, in the presenceof steam, at a temperature of about 649° C. This catalyst was identifiedas catalyst A.

EXAMPLE 2

This example describes the preparation of a dehydrocyclodimerizationcatalyst according to the invention. About 100 cc of Catalyst A wereadded to a round bottom flask equipped with a condenser and containing500 mL of a 2M ammonium nitrate aqueous solution. This mixture washeated to reflux by means of an oil bath and refluxed for 24 hours. Uponcooling the mixture was filtered and the spheres washed five times with100 mL of deionized water. The washed spheres were dried at 150° C. for3 hours and then calcined at 540° C. for 2 hours. This catalyst wasidentified as catalyst B.

The wash water from the ammonium nitrate treatment was analyzed todetermine what elements were present. The analysis of the wash water andcatalyst B are presented in Table A.

                  TABLE A                                                         ______________________________________                                                     Catalyst B                                                                              Wash Water                                             Element      (wt. %)   (wt. %)                                                ______________________________________                                        Si           23.33     0.05                                                   Al           13.50     0.188                                                  P            10.96     0.108                                                  Ga           1.02      0.00003                                                ______________________________________                                    

As Table A shows, the species which are removed by the ammonium nitratewash are composed primarily of aluminum and phosphorus.

EXAMPLE 3

The following test procedure was used to evaluate thedehydrocyclodimerization activity of catalysts. A feedstock of propanewas flowed through a reactor containing the catalyst to be tested. Thepropane was flowed through the reactor at a liquid hourly space velocityof 0.8 hr⁻¹ under a pressure of 1 atmosphere and at a reactor inlettemperature of 540° C. The conversion of propane to aromatics wascalculated at various times during the testing.

Catalysts A and B were accelerated aged by treating the catalysts with ahydrogen/methane gas feed at 1 atmosphere and 565° C. for a period of100 hours. The function of the methane is to act as a diluent. Afterthis hydrogen treatment, the catalysts were oxidized in air at 565° C.and then tested as described above. This hydrogen treatment is believedto be analogous to about one month of on stream operation. The resultsof the testing are presented in Table B.

                  TABLE B                                                         ______________________________________                                        Effect of NH.sub.4 NO.sub.3 Treatment on the Activity of Aged Catalysts                   Conversion (%)                                                                  Catalyst A* Catalyst B*                                         Time (Hrs)    (No Treatment)                                                                            (Treated)                                           ______________________________________                                        11            69.9        78.3                                                23            60.4        68.8                                                35            54.8        60.1                                                47            N/A         N/A                                                 ______________________________________                                         *Aged for 100 hours with H.sub.2 /CH.sub.4 at 565° C.             

The results presented in Table B show that a catalyst that has beentreated with ammonium nitrate has a much higher conversion than one thathas not been treated. Therefore, Catalyst B is much more tolerant tohydrogen exposure than Catalyst A.

We claim as our invention:
 1. A catalyst for converting C₂ to C₆aliphatic hydrocarbons to aromatics comprising a zeolite having a Si:Alratio greater than about 10 and a pore diameter of about 5-6 Å, agallium component and an aluminum phosphate binder, the catalystcharacterized in that it is tolerant to exposure to hydrogen at atemperature of about 500° to about 700° C.
 2. The catalyst of claim 1where the zeolite is a ZSM family zeolite.
 3. The catalyst of claim 1where the zeolite concentration ranges from about 30 to about 90 weightpercent of the catalyst.
 4. The catalyst of claim 1 where the galliumcontent on the catalyst varies from about 0.1 to about 5 weight percent,as the metal, of the catalyst.
 5. A process for preparing a catalyst forconverting C₂ to C₆ aliphatic hydrocarbons to aromatics comprising: a)forming particles from a mixture of a zeolite and an aluminum phosphatebinder; b) calcining said particles at a temperature of about 500° toabout 700° C. and for a time of about 1 to about 15 hours; c)impregnating the calcined particles with a gallium salt; d) heating thegallium containing particles in hydrogen at a temperature of about 500°to about 700° C. for a time of about 1 to about 15 hours, followed byheating in an air/steam mixture at a temperature of about 400° to about700° C. for a time of about 1 to about 10 hours; e) treating theparticles of step (d) with an aqueous solution of a weakly acidicammonium salt or a dilute acid solution at a temperature of about 50° toabout 100° C. for a time of about 1 to about 48 hours; and f) calciningthe particles at a temperature of about 500° to about 700° C. for a timeof about 1 to about 15 hours, thereby providing said catalyst.
 6. Theprocess of claim 5 where the gallium content on the catalyst varies fromabout 0.1 to about 5 weight percent, as the metal, of the catalyst. 7.The process of claim 5 where the zeolite concentration varies from about30 to about 90 weight percent of the catalyst.
 8. The process of claim 7where the zeolite concentration varies from about 50 to about 70 weightpercent of the catalyst.
 9. The process of claim 5 where the zeolite isa ZSM family zeolite.